The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface, and a drill string extending from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. In addition to this conventional drilling equipment, the system also relies on some sort of drilling fluid, in most cases a drilling “mud” which is pumped through the inside of the pipe, which cools and lubricates the drill bit and then exits out of the drill bit and carries rock cuttings back to surface. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well away from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) a drill bit; 2) a steerable downhole mud motor of rotary steerable system; 3) sensors of survey equipment (Logging While Drilling (“LWD”) and/or Measurement-while-drilling (MWD)) to evaluate downhole conditions as well depth progresses; 4) equipment for telemetry of data to surface; and 5) other control mechanisms such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a metallic tubular.
As an example of a potential drilling activity, MWD equipment is used to provide downhole sensor and status information to surface in a near real-time mode while drilling. This information is used by the rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, locations of existing wells, formation properties, and hydrocarbon size and location. This can include making intentional deviations from an originally-planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real time data during MWD allows for a relatively more economical and more efficient drilling operation.
Known MWD tools contain essentially the same sensor package to survey the well bore but the data may be sent back to surface by various telemetry methods. Such telemetry methods include but are not limited to the use of hardwired drill pipe, acoustic telemetry, use of fibre optic cable, Mud Pulse (MP) telemetry and Electromagnetic (EM) telemetry. The sensors are usually located in an electronics probe or instrumentation assembly contained in a cylindrical cover or housing, located near the drill bit.
Mud Pulse telemetry involves creating pressure waves in the drill mud circulating inside the drill string. Mud is circulated from surface to downhole using positive displacement pumps. The resulting flow rate of mud is typically constant. The pressure pulses are achieved by changing the flow area and/or path of the drilling fluid as it passes the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the drilling fluid. The pressure differentials or pulses may be either negative pulse or positive pulses. Valves that open and close a bypass stream from inside the drill pipe to the wellbore annulus create a negative pressure pulse. All negative pulsing valves need a high differential pressure below the valve to create a sufficient pressure drop when the valve is open, but this results in the negative valves being more prone to washing. With each actuation, the valve hits against the valve seat and needs to ensure it completely closes the bypass; the impact can lead to mechanical and abrasive wear and failure. Valves that use a controlled restriction within the circulating mud stream create a positive pressure pulse. Some valves are hydraulically powered to reduce the required actuation power typically resulting in a main valve indirectly operated by a pilot valve. The pilot valve closes a flow restriction which actuates the main valve to create a pressure drop. Pulse frequency is typically governed by pulse generator motor speed changes. The pulse generator motor requires electrical connectivity with the other elements of the MWD probe.
In typical MWD tools, as well as other downhole tools, there are several electrical connections in the tools. Those skilled in the art will be familiar with the different types of electrical connectors commercially available for MWD and other downhole tools. The electrical connectors serve to electrically and/or communicatively couple two or more electrical devices together. The electrical connectors can vary from simple single-pin to complex multi-pin configurations and for downhole use should maintain stability and mechanical strength under downhole conditions. In many cases, electrical connections between components of a tool are configured such that a wire harness (electrical wires in bundle or pigtail) is engaged within the core of the tool, anchored at two ends with plug in connectors. By combining many wires and cables into such a harness, it can provide more security against the adverse effects of vibrations, abrasions, and moisture and reduce the risk of a short. In assembly, the wire harness can have considerable leeway within the bore of the tool and this free space allows the wires to flex, bend and vibrate as they are not secured throughout their length. Over time, the wire harnesses experience torsional and flexural fatigue which can jeopardize the function of the electrical connections. In many cases, a “snubber assembly” is incorporated in the transition between sections of tool where the electrical connectors are placed to assist in reduction or mitigation of the shock and vibration the electrical wire harness is subject to. Snubber devices in general are rubber or metal devices used to control the movement of electronic and electromechanical equipment during abnormal dynamic conditions and typical allow for free movement of a component during normal operation, but dampen shock to the component in an abnormal condition. In addition, centralizers are typically placed around the probe housing where the wire harnesses are contained within, to try to dampen some of the vibration. In downhole environments such as for directional drilling with increased temperature, shock and vibration there are still considerable failures associated with the looseness of the wire harness within the sub-assemblies. There is a high degree of failure of both the coupling devices as well as the electrical connectors so these must be routinely replaced in the downhole tools.
Typically in MWD probes which carry out mud pulse telemetry, measurement of pressure is important for optimizing drilling parameters. Some solutions have targeted the pressure transducer placement within its own separate probe; the probe tends to contain an intricate wire harness but still allows for fluid flow for data telemetry. Sometimes the transducer is exposed to the drilling fluid, which can cause erosive or corrosive failure of the transducer.
There remains a need for appropriate placement and reliable protection of downhole pressure transducers since accurate measurement of pressure in the localized downhole environment is important for efficient drilling.